The present invention generally relates to noise reduction circuits for video signal, and more particularly to a noise reduction circuit which can effectively eliminate a noise component existing in a luminance signal of a reproduced video signal.
Generally, a noise component is mixed in a video signal which is reproduced from a recording medium in a video signal recording and reproducing apparatus such as a magnetic recording and reproducing apparatus (so-called video tape recorder) which plays a magnetic tape. However, in a recording system of the magnetic recording and reproducing apparatus, especially a high frequency component of a luminance signal of the video signal which is recorded and reproduced is subjected to a pre-emphasis and then to a frequency modulation. For this reason, the frequency of a frequency modulated signal especially at a part immediately after a part (rise) where the luminance signal rapidly changes from a black level to a white level and at a part immediately after a part (fall) where the luminance signal rapidly changes from a white level to a black level is high because of the pre-emphasis carried out in the recording system. But in a general magnetic recording and reproducing apparatus, a signal-to-noise (S/N) ratio of a recording and reproducing characteristic is poor in a high frequency range. Hence, the S/N ratio is poor at the parts immediately after the rise and immediately after the fall in the luminance signal, and it is desirable to positively eliminate the noise component existing at such parts of the luminance signal.
As one example of the conventional noise reduction circuit, there is a first noise reduction circuit comprising a lowpass filter, first and second subtracting circuits and a limiter. An input luminance signal including a noise component is supplied to the lowpass filter and to the first and second subtracting circuits. An output signal of the lowpass filter is subtracted from the input luminance signal in the first subtracting circuit, and an output signal of the first subtracting circuit is supplied to the limiter. An output signal of the limiter is subtracted from the input luminance signal in the second subtracting circuit, and an output signal of the second subtracting circuit is obtained as an output of the noise reduction circuit.
However, as will be described later on in the present specification in conjunction with the drawings, the first noise reduction circuit suffers a disadvantage in that it is impossible to eliminate the noise component existing at the parts immediately after the rise and immediately after the fall in the luminance signal. The S/N ratio is poor at the parts immediately after the rise and immediately after the fall in the luminance signal as described before, and it is desirable to positively eliminate the noise component existing at such parts of the luminance signal.
On the other hand, as another example of the conventional noise reduction circuit, there is a second noise reduction circuit previously proposed in a U.S. Pat. No. 4,652,922 filed Dec. 26, 1985 in which the applicant is the same as the applicant of the present application. The second noise reduction circuit comprises first and second circuit parts each having a circuit construction similar to that of the first noise reduction circuit described before. The first and second circuit parts are coupled in series so that the output signal of the first circuit part (that is, the output signal of the second subtracting circuit in the first circuit part) is supplied to the second circuit part (that is, the lowpass filter in the second circuit part), and the output signal of the second subtracting circuit in the second circuit part is obtained as an output of the second noise reduction circuit. The lowpass filter in the second circuit part has an impulse response characteristic complementary to that of the lowpass filter in the first circuit part.
As will be described later in conjunction with the drawings, the second noise reduction circuit can eliminate the noise component existing at the parts immediately after the rise and immediately after the fall in the luminance signal. However, although the noise component is satisfactorily eliminated, there is a problem in that the waveform of the luminance signal becomes distorted in the vicinities of the rise and fall thereof when the noise reduction is carried out in the second noise reduction circuit.